The typical digital phase and frequency modulator (PM/FM) suffers from one or more inherent performance and/or implementation limitations. One conventional approach for digital phase and frequency modulation first generates a relatively low frequency modulated signal using a low to moderate sample rate digital to analog converter (DAC) and an anti-aliasing filter. The resulting signal is then up-converted to the desired output frequency using one or more frequency translation circuits. The disadvantages of this approach are the complexity of the frequency translation circuit implementation using mixers and filters, isolation and bandwidth limitations associated with frequency translation filtering, and the presence of mixing spurious signals at the final output.
Another similar conventional implementation generates quadrature low frequency modulated signals using two digital-to-analog converters and anti-aliasing filters with a low to moderate sample rate. These signals are used to control a quadrature modulator, which directly produces the modulated output centered at the frequency of its local oscillator input. This approach provides lower complexity and wider output bandwidth capabilities than the up-conversion approach. However, the major disadvantage of this approach is that it produces distortions due to amplitude and phase imbalances which are unacceptable in many applications.
A third conventional approach uses an undersampling (or bandpass sampling) technique with a digital-to-analog converter with a moderate to high sample rate. The output signal is a sampling image which is a sum or difference of the fundamental digital-to-analog converter output frequency and a harmonic of the sampling frequency. The image is selected from all of the other images present at the digital-to-analog converter output with a bandpass filter. This approach has bandwidth limitations due to the output filtering, and dynamic range (signal-to-noise ratio) limitations of the digital-to-analog converter at output frequencies above its sample rate.
A fourth conventional implementation utilizes a very high speed digital-to-analog converter which oversamples the desired output frequency. This approach avoids the frequency conversion and filtering limitations of the other aforementioned implementations. However, spurious signals are present in the output of this implementation due to the limited resolution of the high speed digital-to-analog converter. The high levels of the spurious signals are unacceptable in many applications. The circuitry required to generate the high speed digital-to-analog converter data inputs also generally has much higher complexity and power consumption than the other approaches.